Limitations on MLC Flash Page Reuse and its Effects on Durability

Flash memory is prevalent in modern servers and devices. Coupled with the scaling down of flash technology, the popularity of flash memory motivates the search for methods to increase flash reliability and lifetime. Erasures are the dominant cause of flash cell wear, but reducing them is challenging because flash is a write-once medium— memory cells must be erased prior to writing. An approach that has recently received considerable attention relies onwrite-once memory (WOM) codes, designed to accommodate additional writes on write-once media. However, most techniques proposed for reusing flash pages with WOM codes are limited to SLC flash, and are not applicable to MLC flash due to the specific constraints it imposes. In this study, we use a hardware evaluation platform to identify the limitations of reprogramming flash pages on state-of-the-art MLC flash chips, and to directly measure the short and long-term effects of page reuse on SSD durability and energy consumption. Our findings provide key guidelines for FTL designs that employ WOM codes for reusing flash pages.